1. Field of the Invention
The present invention relates to a process for producing a polyolefin. More particularly, it pertains to a process for efficiently producing a polyolefin, particularly a linear low-density polyethylene in the presence of a catalyst having a high polymerization activity in a reduced amount of an aromatic hydrocarbon solvent which is harmful to health without using an expensive metallocene compound as the polymerization catalyst.
2. Description of Related Art
In recent years, in a process for producing a polyolefin, a catalyst comprising a metallocene compound of a transition metal and an aluminoxane has been suggested as a novel uniform catalyst (Japanese Patent Application Laid-open No. 19309/1983). This uniform catalyst has a very high activity and an excellent copolymerizability, but the metallocene compound and the aluminoxane are more expensive as compared with a conventional Ziegler-Natta catalyst. Thus, the suggested catalyst has a problem that its cost is high.
In order to overcome the above-mentioned drawback, there has been suggested a method in which a uniform catalyst comprising an oxygen-containing titanium compound and an aluminoxane is used without employing any metallocene compound (Japanese Patent Application Laid-open No. 3008/1988). However, this method also has a problem that a catalytic activity is so low that a polyolefin cannot be efficiently produced.
Furthermore, in the above-mentioned catalyst system using the aluminoxane, a large amount of an aromatic hydrocarbon such as toluene is used as a polymerization solvent or a catalyst control solvent, but the aromatic hydrocarbon has a problem of carcinogenicity or the like. For this reason, it is desired to use another safe polymerization solvent or catalyst control solvent.
The group of the present inventors has already suggested, as a technique for solving the above-mentioned problem, a process using a catalyst which mainly comprises a specific transition metal compound, an aluminoxane, an organic aluminum compound and/or a compound having a .pi. electron (Japanese Patent Application No. 234077/1992).